The formation of high-quality thin III-nitride layers poses a demanding problem in the context of recent high-performance semiconductor devices, for example, transistor devices and optoelectronic devices, in particular, laser devices such as vertical cavity surface-emitting lasers (VCSELs).
Conventionally, a high-quality thin III-nitride layer is formed by epitaxial growth on a seed substrate, for example, a sapphire substrate, by molecular beam epitaxy (MBE), metalorganic vapour phase epitaxy or hybrid vapour phase epitaxy. Epitaxial growth on the seed substrate commonly involves the growth of a thick III-nitride layer, commonly referred to as a buffer or buffer layer, prior to growth of the high quality thin III-nitride layer, wherein the thick III-nitride buffer layer promotes an improvement in the quality of the III-nitride material of the thin layer.
The grown layer can subsequently be transferred to a target wafer by some wafer transfer process known in the art, for example, bonding and laser lift-off or the SMART Cut™ process. Thinning of the III-nitride layer(s) to produce the high quality thin III-nitride layer can be achieved by removal of the thick buffer layer after the wafer transfer. Such a thinning process can be achieved, for example, by grinding, polishing or plasma etching the layer(s). Controlled uniform thinning of the III-nitride material can be complex due to the thickness of the buffer layer and this can impact the thickness uniformity of the thin high quality III-nitride layer.
The defect density of the transferred III-nitride layer proves crucial for the performance characteristics of the eventually finished semiconductor device. A main reason for the defects can be seen in strain induced in the III-nitride layer due to growth on a layer exhibiting a different crystal structure, i.e., a mismatch of the crystal lattice constant of the III-nitride layer and the seed layer whereupon it is epitaxially grown.
In addition, the strain state and polarity of the transferred III-nitride layer can prove crucial for performance characteristics of the eventually finished semiconductor device. Such characteristics may include, for example, the carrier recombination efficiency. Strain between the transferred III-nitride layer and a subsequent active layer can affect internal piezoelectric fields within the semiconductor device. Furthermore, the polarity of the transferred III-nitride layer can also affect the piezoelectric fields within the semiconductor structure and polarity selection may be utilized in order to mitigate the affect of strain on the internal electric fields.
Thin III-nitride layers represent the main elements of recent down-scaled optoelectronic devices, particularly, laser devices such as vertical cavity surface-emitting lasers (VCSELs). U.S. Pat. No. 6,320,206 B1 discloses a method for the manufacture of a VCSELs having wafer bonded aluminum-gallium-indium-nitride structures and mirror stacks. According to the teaching of U.S. Pat. No. 6,320,206 B1, p- and n-doped InAlGaN layers are grown on a sapphire substrate and subsequently transferred to a target substrate. In the resulting structure the InAlGaN layers are sandwiched by mirror layers to obtain a VCSEL.
Despite the recent engineering progress, however, there is still a need for providing III-nitride layers with enhanced uniformity and reduced defect density in general and, in particular, improved VCSELs based on III-nitride layers. The invention now satisfies this need and provides such improved prodcuts and structures.